This is actually an interesting and subtly presented bit of information
about Animagi. If an
Animagus has some
physical disability in human form - at least, in the nature of missing
a part of his or her body - that disability is reflected in the
Animagus form.
Pettigrew is missing a finger,
so his rat form is missing the corresponding toe on his front paw.

"...he stole the passwords into Gryffindor Tower for me...
As I understand it, he took them from a boy's bedside table."

Implicit in this statement is an interesting degree of communication between
Sirius and the remarkable
Crookshanks, as well as of
Crookshanks' own
understanding of humans. Sirius
could've figured out what the password list was when
Crookshanks brought it to
him, but the detail that it came from a boy's bedside table must have been
conveyed by Crookshanks.
(After all, Crookshanks'
human companion is a girl, not a boy;
Sirius had no reason we know of to
suppose that Crookshanks got
the passwords from a boy, nor to guess the detail of the bedside table.)

All we can really deduce from Sirius'
remark is that he and Crookshanks
were able to communicate with one another; the details of how this took
place are still a matter for speculation. Having said that, I will proceed
to speculate.

Note that Crookshanks was able to understand what the list of passwords
was, although we are not told how he knew. This may suggest that he can
understand human speech, but not necessarily. Possibly as a part-kneazle
he has some Legilimency-like
ability to perceive human thoughts and/or emotions, as kneazles are noted in
FB as having an uncanny ability to read
human character, but this is only a hypothesis, not a fact. If
Crookshanks were to have
such an ability, it might also account for his ability to communicate with
Sirius, particularly if
Sirius were in Animagus form at
the time and his mind in the accompanying altered state of consciousness.

Another factor presented later in the series are Voldemort's remarks
to the Death Eaters on the subject of
Wormtail in
GF33, regarding his
"curious affinity with rats".
Implicit in those remarks is the information that
Pettigrew could communicate
with rats to obtain information involving rather complex concepts. It seems
a reasonable deduction that
Pettigrew's rat
Animagus status is a factor in
his ability to communicate with real rats.

"Force him to show himself," said Lupin. "If he really is a rat,
it won't hurt him."

So it is possible to force an Animagus
to return to his or her human form. This spell requires the use of
wands, as shown in the text, but
requires no incantation, and we do not know what the spell is called.

It was like watching a speeded-up film of a growing tree.

Despite the fact that
Animagus transformations
have occurred "on stage" twice before at this point in the series
(PS1,
PA6), this is the first
detailed description we have had of the process from an onlooker's point
of view.

"Voldemort's been in hiding for fifteen years, they say he's half dead."

This is actually a FLINT in the first U.S. edition; Voldemort had been
in hiding only since the incident in Godric's Hollow when Harry was a year
old, and Harry is now only a few weeks short of his fourteenth birthday.
The number should be twelve, not fifteen.

"They could tell that my feelings were less - less human, less complex
when I was a dog...but they thought, of course, that I was losing my mind
like everyone else in there..."

From Whisp's remarks in
QA1 and observations of
Animagi throughout the series, we
observe that Animagi retain their
faculties of reason even when in non-human form. This remark by
Sirius is the first indication that
the transformation brings with it a somewhat altered state of consciousness
despite the retention of the capacity for intelligent thought.

This is the first time in the entire conversation that
Pettigrew has referred to
Voldemort as the 'Dark Lord'
rather than the more neutral 'He-Who-Must-Not-Be-Named'. He and
Trelawney - and
Harry when quoting
Trelawney's prophecy - are the
only people in the book who use either term to refer to
Voldemort.

Interesting that Pettigrew only
slipped into this form of referring to
Voldemort after he knew that he
had no chance of talking his way out of the accusation of being a
Voldemort supporter.

"YOU'D BEEN PASSING INFORMATION TO HIM FOR A YEAR BEFORE
LILY AND JAMES DIED! YOU WERE HIS SPY!"

In light of this information, consider Moody's litany of the names
of the members of the Order of the Phoenix killed during the first war
against Voldemort - the period when Pettigrew was both a member of the
Order and spying for Voldemort (OP9).

It would be interesting to know how many of those deaths, apart from those of
Lily and
James, could be put down to
Pettigrew's account.

The depth of Snape's and
Sirius Black's mutual dislike is
shown, as Snape prepares to turn
Sirius over to the
Dementors without further ado,
despite the protests of four other people present.

Pettigrew's confession, which
has to be squeezed out of him point by point despite the damning fact that
he's been alive and in hiding all this time.

Harry's determination - to the
point of blocking Pettigrew
with his own body - that his father's best friends shouldn't become killers
just for Pettigrew's sake.

Harry is shown thinking of
Sirius Black as
'Black' throughout this chapter.
Only in the next chapter, after he has been convinced of
Sirius' innocence,
does he begin thinking of his godfather by his given name.

Ron taking the revelation of
Scabbers' true identity as a
personal insult. For this, he became estranged from
Hermione for so many weeks.

"Well, hello, Peter," said Lupin pleasantly, as though rats frequently
erupted into old school friends around him. "Long time, no see."
(One of MLW's personal favourites, particularly on the
Jim Dale recording of the book. Is Remus Lupin seriously cool, or
what? -- MLW

"I've heard them screaming all sorts of things in their sleep."

"I must admit, Peter, I have difficulty in understanding why an innocent
man would want to spend twelve years as a rat," said Lupin evenly.